ABSTRACT Bullying, sexual violence, and dating violence among adolescents are all major public health problems that occur at relatively high rates and demand attention to alleviate the considerable suffering they cause. These problems share developmental correlates and evidence is emerging that bully perpetration and victimization is concurrently and longitudinally associated with sexual harassment, gender-based harassment, and teen dating violence involvement (Basile et al., 2009; Espelage et al., 2012). Despite the costs of bullying, the impact of prevention programs in the US has been disappointing, especially in middle-schools. Social-emotional learning (SEL) programs are reducing aggression in US schools (Durlak et al., 2011), in part, by helping students develop social and emotional skills to manage conflicts, but effects could be greater. Data presented in this application strongly suggest that to increase effects, school-based bullying prevention programs need to: 1) integrate evidence-based approaches that focus on different levels of influence (individual, peer, school), 2) focus on gender-based harassment and violence (i.e., sexual harassment and violence, dating violence, harassment and violence associated with sexual orientation and/or gender-role nonconformity) and 3) address a major driver of bullying and gender-based harassment and violence - traditional masculine ideology and homophobic name-calling. Thus, this application proposes a large-scale RCT comparing the Second Step (CfC, 2008) program to a gender-enhanced Second Step + Shifting Boundaries program (SB; Taylor et al., 2011). The SB program combines a classroom curriculum that addresses sexual harassment, gender-based harassment, and dating violence with whole-school strategies to decrease these outcomes. School-wide strategies include school protocols for responding to dating violence and sexual harassment and increased monitoring of hot spots. Twenty-eight middle schools (grades 6 - 8) from two school districts in Illinois will be randomly assigned to either the Second Step only condition or the gender-enhanced Second Step condition (Second Step/SB). Two cohorts (6th and 7th graders) will complete baseline and follow-up surveys. Study aims are to evaluate the differential efficacy of the Second Step: Student Success Through Prevention program (Second Step) versus a gender-enhanced Second Step program (Second Step/SB) on reducing bullying, sexual harassment, gender-based harassment, and teen dating violence; to assess the extent to which gender-enhanced Second Step versus Second Step results in greater increases in positive bystander intervention around bullying, sexual harassment, gender-based harassment, and teen dating violence; and to test the extent to which the intervention impacts peer-level attitudes toward bullying, sexual harassment, homophobic name-calling, and teen dating violence through its effect on peer social dynamics. This study is highly innovative and could have substantial public health impact by targeting bullying, dating violence, and sexual harassment.